standard hydrogen electrode (SHE) is the primary reference electrode in electrochemistry and forms the basis for the measurement of redox potentials. This electrode is known as the zero-potential reference electrode against which all the other electrodes are measured. In this comprehensive article, we will detail the structure, function, applications, and importance of the standard hydrogen electrode.
History and Basic Principles
Discovery and Development
concept of hydrogen electrode was first proposed by pioneering chemists, such as Walter Nirenst, in the late 19th century. In 1900, Niernst proposed the famous hydrogen electrode equation, which describes the relationship between the electrode potential and the chemical activity of reactants.
Scientific Definition
A standard hydrogen electrode is defined as a platinum electrode exposed to a single atmospheric pressure of hydrogen gas, immersed in an aqueous solution with a pH of 1 (pH = 0). In these conditions, the electrode potential is considered to be equal to zero at any temperature.
Structure and Structure of Standard Hydrogen Electrode
1. Platinum Tycoon
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It is made of pure platinum and has a black platinum finish.
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Black platinum provides a very large active surface area, which is essential for hydrogen absorption and reaction.
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electrodes are usually in the form of flat sheets or coils.
2. Hydrogen chamber
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Use high-purity hydrogen (usually 99.999%) with a precise pressure of 1 atmosphere.
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The system should be completely free of impurities, especially oxygen.
3. Electrolyte
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Hydrochloric acid (HCl) is generally used, which has a hydrogen ion activity of 1.
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In practice, a concentration of about 1.18 M of HCl is used to achieve these conditions.
4. Standard Temperature
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Although the SHE potential is defined as zero at all temperatures, measurements are typically made at 25 degrees Celsius (298.15 Kelvin).
Mechanism of Electrochemical Reaction
Half Electrode Reaction
The main reactions at the standard hydrogen electrode are as follows:
2H⁺(ac) + 2e⁻ ⇌ H₂(g)
Equilibrium conditions
In order for this pole to serve as a reference, the following conditions must be met:
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The partial pressure of H₂ is exactly 1 atm.
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activity of the H⁺ ion is exactly 1
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Platinum should be completely pure and active.
How to prepare a standard hydrogen electrode
Step 1: Prepare the Platinum Electrode
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Polishing the platinum surface with very fine sandpaper.
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Clean the electrode in a concentrated acidic solution (usually HNO₃).
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Electrolysis of electrodes is performed to form black platinum.
Step 2: Hydrogen Saturation
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Place the electrodes in the acidic solution.
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Pure hydrogen passes through the reactor at a controlled flow rate.
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This process continues until the electrode is fully saturated.
Step 3: Calibration and Inspection
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The electrodes are connected to the measurement system.
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This potential is measured relative to a second reference electrode.
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Final adjustments will be made if necessary.
Applications of Standard Hydrogen Electrode
1. Standard electrode voltage measurement.
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All standard reduction potentials are measured relative to SHE.
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It is on this basis that the standard potentiometer is created.
2. Thermodynamics
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Calculate Gibbs’ free energy for electrochemical reactions.
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Determination of the equilibrium constant of redox reaction
3. Research Corrosion
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Measuring the corrosion potential of different materials.
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Investigation of the Corrosion Mechanism
4. Battery and fuel cell industry
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Design and Optimization of Energy Storage Systems
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Study the performance of electrodes in different environments.
5. Research in Electrobiochemistry
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Research into biological systems involving electron transport
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Research into Electrolytically Active Enzymes and Proteins
Advantages and Disadvantages of Standard Hydrogen Electrode
Feature:
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Absolute reference with specific zero potentials
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Repeatability and high reliability
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Fast and stable response
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It can be used in a wide range of temperatures.
Limit:
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Preparation is complex and time-consuming.
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Precise control of hydrogen pressure requires special equipment.
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Sensitivity to impurities
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Difficulty in continuous use and transfer
Alternative Reference Pole
Due to practical difficulties in using SHE, secondary reference electrodes are used in many applications:
1. Saturated Calomel Electrode (SCE)
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Based on Hg/Hg₂Cl₂
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The relative potential of SHE is approximately +0.242 V
2. Silver chloride electrode (Ag/AgCl)
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Depends on Ag/AgCl
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The relative potential of SHE is approximately +0.197 V
3. Copper electrode and copper sulfate
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Soil and Erosion Research
Relevant International Standards
The use of a standard hydrogen electrode is defined by recognized international organizations:
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International Union of Pure and Applied Chemistry (IUPAC )
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ASTM (American Society for Testing and Materials)
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ISO (International Organization for Standardization)
Safety tips when working with SHE
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Hydrogen : A highly flammable substance that requires a proper ventilation system.
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Strong acids: Personal protective equipment should be used.
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Black Platinum: Avoid contact with or inhalation of the skin.
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Closed system: ensures no hydrogen leakage.
New Research and Future Developments
1. Solid-state reference electrode
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Developing a more stable reference pole
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Decline in demand for hydrogen
2. Microelectrode System
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of Reference Electrodes at the Micro and Nanoscale
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Applications in Medical Devices and Sensors
3. New rules
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Explore the possibility of revising the standard definition
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Adapting to new technologies
Conclusion
modular hydrogen electrode is a cornerstone of modern electrochemistry and plays a vital role in scientific and industrial research. Although its practical application in the laboratory is complex, understanding its principles and mechanisms of action is crucial for any chemist, materials engineer or related scientific professional. As technology has advanced, we have seen the development of new reference systems, but the standard hydrogen electrode (SHE) remains the ultimate gold standard for electrochemical measurements.
Frequently asked questions
1. Why is platinum used in a standard hydrogen electrode?
Platinum was chosen as an electrode material for its excellent catalytic properties, chemical stability, and corrosion resistance. Black platinum provides a very large active surface area, which is essential for hydrogen absorption and reaction.
2. Can other metals be used instead of platinum?
Other precious metals, such as palladium, but platinum is preferred for its better performance and low reactivity. Base metals, such as copper or iron, are not suitable.
3. Why should the pressure of hydrogen be equal to exactly one atmospheric pressure?
This condition is derived from the thermodynamic definition of a standard voltage. Any deviation from this pressure results in a change in the electrode potential, which can be calculated using the Niernst equation.
4. How does a standard hydrogen electrode (SHE) behave at different pH values?
The standard hydrogen electrode potential is zero only at pH = 0 (hydrogen ion activity = 1). At other pH values, its potential follows the Nirenst equation and cannot be used as a standard reference.
5. What is the accuracy of the SHE measurement?
Under ideal conditions, and with precise control of the coefficients, an accuracy of ±0.001 volts can be achieved. However, the actual measurement accuracy is usually around ±0.005 volts.